python: pie menu to display the pivot center modes
It already is an operator.

There is no need to create another operator to call an operator, so it can be given a shortcut. 

Can add a keymap to it directly. The field to data_path ignore if appears disabled, select and type in the required RNA path from context.


shortcut set to  &#8679; Shiftz (disabled the toggle render shading)

or created via code (example) 

import bpy

wm = bpy.context.window_manager
kc = wm.keyconfigs.user

km = kc.keymaps['3D View']
kmi = km.keymap_items.new('wm.context_pie_enum', 'Z', 'PRESS', shift=True)
kmi.properties.data_path = "space_data.pivot_point"


Alternatively it can be added as an expanding pie menu item in your original question code via

op = pie.operator("wm.context_pie_enum")
op.data_path = "space_data.pivot_point"


To call an operator from code, all operator properties must be passed as  keyword arguments

bpy.ops.wm.context_pie_enum(data_path="space_data.pivot_point")  


The first argument of an operator call can be a string if it is one of the operator context enums, eg 'INVOKE_DEFAULT'. See error message in question for full list.  Another type of argument is a dictionary of context override members.

bpy.ops.wm.context_pie_enum('INVOKE_DEFAULT', data_path="space_data.pivot_point") 
c = {"area": context.screen.areas[4]} # will need more than just area.
bpy.ops.wm.context_pie_enum(c, data_path="space_data.pivot_point")